Morphology and Staining Properties
The causative agent of leptospirosis (L. interrogans) belongs to the family Leptospiraceae. They are thin spirochetes, 6–24 µm in length and no more than 0.15 µm in diameter.
- Structure: The cell has 20 to 40 primary coils. The ends of the bacterium are curved, resembling hooks. Spores and cysts are not formed.
- Motility: Provided by single periplasmic flagella (axial filaments) originating from each pole of the cell.
- Staining: Leptospires take up aniline dyes extremely poorly and are difficult to distinguish with Gram staining or Wright-Giemsa stain. The optimal visualization method is dark-field microscopy or phase-contrast microscopy.
Cultural Properties and Resistance
Leptospires are aerobes and grow slowly. The optimal temperature for their multiplication is 28–30 °C, which is lower than normal human body temperature.
Lipids—long-chain fatty acids and alcohols—serve as their energy and carbon source. Cultivation on artificial media strictly requires the addition of serum albumin or blood serum. In liquid media, they multiply by transverse fission, without causing turbidity (growth remains transparent).
Environmental Resistance:
| Factor | Effect on Leptospires |
|---|---|
| Drying and low pH | Rapidly killed |
| Heat | Destroyed at 56 °C in 25–30 minutes, instantly upon boiling |
| Wet environment (water bodies) | Remain viable for up to 30 days |
| Moist soil | Can survive long-term—up to 280 days |
Epidemiology and Pathogenesis
Leptospirosis is a natural focal zoonosis with a characteristic summer-autumn seasonality. Wild and synanthropic rodents serve as the primary reservoir, while domestic animals (dogs, pigs, cattle) act as secondary reservoirs. In animals, the infection proceeds chronically and asymptomatically: bacteria are shed in urine, contaminating soil and water.
Humans are highly susceptible but represent a "dead end" for the infection—the pathogen is practically never transmitted from a sick person to a healthy one.
Phases of Infection Development:
- Portal of entry: Intact mucous membranes of the gastrointestinal tract or skin microtraumas (waterborne transmission predominates).
- Primary dissemination: Penetration into the bloodstream and seeding of organs of the reticuloendothelial system (kidneys, liver).
- Incubation period: Lasts 7–10 days; bacteria actively multiply in parenchymal organs.
- Secondary bacteremia: Massive release of leptospires and their toxins into the blood, which coincides with the acute onset of clinical manifestations.
- Organopathology: Bacteria damage capillaries, causing hemorrhagic syndrome, jaundice, aseptic meningitis, and renal failure. Case fatality rates range from 3% to 40%.
Diagnosis, Treatment, and Prevention
Biological samples for laboratory testing (blood, urine, cerebrospinal fluid, serum) are chosen depending on the stage of the disease.
- Microbiological diagnosis: Includes dark-field microscopy, culture on nutrient media, and PCR.
- Serology: The gold standard is the microscopic agglutination test (MAT) using a panel of reference live cultures. ELISA is also used.
- Biological assay: Performed by inoculating suckling rabbits.
Treatment: The basis of etiotropic therapy is antibiotics of the penicillin and tetracycline groups. Additionally, specific heterologous antileptospiral immunoglobulin, derived from equine serum, is used.
Prevention: Nonspecific measures include rodent control (deratization) and the vaccination of farm animals. For epidemiological indications, specific prophylaxis in humans is carried out using a heat-killed whole-cell vaccine containing antigens of the four main serogroups.